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09-12-2013 08:06 AM

Dear anyone

I am using SAS 9.4, enterprise guid 6.1. I am looking at the risk of taking medicine X if you have symptom A, B and C. I have to groups: I Use of medicine X and II no use of medicine X. All symptoms are more frequent in the group that uses medicine. When I do univariate analysis I get the following odds ratio for taking medicine X: 3.4, 2.3, and 1.2. When I do a logistic regression I get 0.3, 0.6 and 1.2

I can appreciate that I get something close to the inverse OR depending of my coding. But how can symptom C point in another direction than the rest. To me it does not make any sense. Can anyone please help me.

Se tables for details

Sincerely

Anders

Univariate | ||||

Symp A | Total | |||

0 | 1 | |||

med | ||||

0 | Frequency | 513 | 144 | 657 |

Row Pct | 78.08 | 21.92 | ||

1 | Frequency | 287 | 270 | 557 |

Row Pct | 51.53 | 48.47 | ||

Total | Frequency | 800 | 414 | 1214 |

OR=3.4 | ||||

SympB | Total | |||

0 | 1 | |||

med | ||||

0 | Frequency | 548 | 109 | 657 |

Row Pct | 83.41 | 16.59 | ||

1 | Frequency | 382 | 175 | 557 |

Row Pct | 68.58 | 31.42 | ||

Total | Frequency | 930 | 284 | 1214 |

OR=2.3 | ||||

SympC | Total | |||

0 | 1 | |||

med | ||||

0 | Frequency | 598 | 59 | 657 |

Row Pct | 91.02 | 8.98 | ||

1 | Frequency | 497 | 60 | 557 |

Row Pct | 89.23 | 10.77 | ||

Total | Frequency | 1095 | 119 | 1214 |

OR=1.2 |

Logistic regression | ||||

Odds Ratio Estimates and Wald Confidence Intervals | ||||

Effect | Unit | Estimate | 95% Confidence Limits | |

SympA 0 vs 1 | 1 | 0.334 | 0.257 | 0.435 |

SympB 0 vs 1 | 1 | 0.627 | 0.468 | 0.842 |

SympC 0 vs 1 | 1 | 1.157 | 0.771 | 1.734 |

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09-12-2013 10:57 AM

Code used to generate the summary will be helpfule as there are enough options involved that just the output is not enough for diagnostics.

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10-11-2013 05:00 AM

Dear ballardw

Thank you for your answer. I sincrely appologize for my late reply. I have had some IT difficulties.

I have talked to a statistician and the 'problem' was just confounding.

Sincerely Anders

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09-12-2013 12:08 PM

My first haunch would be that you did not use the DESCENDING option in your proc logistic so it's modeling the odds ratio of going from state "taking the medicine" to "not taking the medicine"

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10-11-2013 05:00 AM

Dear Vince

Thank you for your answer. I sincrely appologize for my late reply. I have had some IT difficulties.

I have talked to a statistician and the 'problem' was just confounding.

Sincerely Anders